1. Field of the Invention
This invention relates to a gas sensor, and more particularly to a gas sensor for detecting a gaseous ingredient or its concentration.
2. Description of the Prior Art
It has been known to detect the presence and concentration of a gas in air by a gas sensor which uses a gas-sensitive element made of an oxide semiconductor, such as tin oxide (SnO.sub.2), zinc oxide (ZnO), titania (TiO.sub.2), cobalt oxide (CoO), and the like, whose electric resistance varies upon exposure to the gas. To simplify the structure of such gas-sensitive element for improving the productivity thereof, the so-called hybrid techniques has been developed; for instance, by printing both the gas-sensitive element and electrodes therefor in the form of thick films on an electrically insulating ceramic substrate.
To hold such a gas sensor in position for actual measurement, it has been practiced heretofore to put it in a housing which can be readily fixed at a location for detecting the object gas. However, certain difficulties have been experienced: for instance, that the gas sensor with the gas-sensitive element formed on the ceramic substrate has smooth surface with little undulations and it has been difficult to determine which part of the sensor be fixed to the housing with or without a spacer therebetween; and that, when a spacer is used between the housing and the gas sensor, the spacer tends to contact the gas-sensitive element and do harm to it during the assembling process, because the gas-sensitive element projects from the substrate for sensing the gas while the spacer keeps rather tight contact with the substrate for ensuring the secure holding.